1. Field of the Invention
This invention relates to a synchronous signal generator for obtaining a system clock, horizontal synchronizing signal and frame synchronizing signal which are synchronized with a MUSE signal in the MUSE system, which is one of the new high definition television systems.
2. Description of the Prior Art
As a high definition television system which is capable of providing higher resolution image than that of the conventional standard television system, the MUSE (Multiple Sub-Nyquist Sampling Encoding) system has been proposed by NHl. In that system, signal is compressed up to about one-third to the 8 MHz frequency band, thereby allowing one channel transmission by a broadcasting satellite. (See "An HDTV broadcasting system utilizing a bandwidth compression technique:MUSE", IEEE Trans. Broadcast. Vol. 33, No. 4, pp.130-160 (1987)) This system can be used not only for satellite broadcasting but also for package media such as video disks, VCR's and the like. Mutual communications between the MUSE system units are effected in the form of analog MUSE signals in general.
Generally, when a signal is sent or received as an analog signal between units whose internal processings are executed digitally, a D/A converter, a low-pass filter and a A/D converter are necessary to be additionally used for transmission. On the other hand, if mutual communications between them are effected directly by a digital signal, the components shown above can be eliminated, thus making it possible to simplify the system as well as to prevent the deterioration of a signal. The same result can be obtained in the case when the MUSE system units mutually communicate.
An explanation follows on the synchronization of the MUSE signal. The MUSE signal does not have a negative synchronizing signal, which can be separated by the amplitude separation as used in the conventional video signal, but rather has a positive synchronizing signal which has a special waveform containing both the frame synchronizing signal and the horizontal synchronizing signal in the amplitude of a video signal. That is, the frame synchronizing signal has a signal waveform having the line correlation and the clock correlation in combination and can be detected by pattern matching techniques. Also, the horizontal synchronizing signal has a signal waveform whose sampling phase error relative to the normal horizontal synchronization position can be detected by computing sampling values at the middle point and both ends of the waveform.
The procedures of the synchronous signal generation of the MUSE signal according to the prior art are explained below. First, an inputted analog MUSE signal is converted through an A/D converter into a digital MUSE signal using a system clock. From the digital MUSE signal thus converted, a frame synchronizing signal is detected by pattern matching. A horizontal synchronizing signal is detected by counting the system clock with the detected frame synchronizing signal used as a reference. Then, the sampling phase error of the detected horizontal synchronizing signal is calculated. The sampling phase error thus calculated is fed through a low-pass filter and a D/A converter back to a voltage-controlled oscillator. An output from the voltage-controlled oscillator is frequency-divided to generate a system clock. The system clock thus generated is used as a system clock of the A/D converter to control through a PLL (Phase Locked Loop) circuit. Thus, the synchronous signal generation can be realized in accordance with the above-mentioned arrangement and procedures.
The conventional synchronous signal generation as explained above is contingent upon the synchronous signal generation of an analog MUSE signal and executes an accurate clock phase synchronization by controlling the phase of a sampling clock when performing the A/D conversion. Thus, if a digital MUSE signal is directly inputted, then the clock synchronization cannot be performed, so that it cannot respond to the synchronous signal generation of the digital MUSE signal. That is, even if the phase of a system clock is changed by controlling the voltage-controlled oscillator in response to the sampling phase error, when an input signal is a digital MUSE signal, the clock timing to an inputted data only can change with no change in the value of the inputted data itself. As a result, the loop control is impossible and the synchronization on a clock by clock basis cannot be performed.